It all depends on the plane you're flying and the average altitude of the fight you are looking to be a part of. Generally speaking, the worse your plane turns the higher you want to be, plane performance at alt taken into account of course. My rule of thumb when in a speed ride is to be higher than 75% of my enemy. If you are underneath an enemy who can also outmanuever your plane, you are going to have problems if he's any good.
If you are in a plane that can outmanuever 75% of your enemy you only need be higher than 25% of your enemy to be even money against someone with equal skill on average. So, if your in a turner altitude is not nearly as important (presuming the 25% you have alt on are the 25% that can out-turn you). In fact, being lower in a turner has the advantage of seducing an otherwise reluctant but higher foe into engaging you when he would likely not do so if you were at or close to his altitude.
Of course, if everyone followed these rules what ends up happening is an 'altitude arms race'. That is, everyone keeps getting higher and higher trying to maintain that 'even-money' situation in their various rides. Thankfully, most do not abide by my guide.

Zazen